Routers are devices that direct traffic in a network. Each router in the network has a route table that typically includes routing information to enable incoming packets to be forwarded or routed to their destination in the network. Some routers forward packets based only on the destination address indicated in the packet. Other, more complex, routers forward or route packets based on policies defined, for example, by a network administrator. The latter routing schemes are commonly referred to as policy-based routing.
Policy-based routing can enable packets to be forwarded or routed in a network based on any number of criteria, including the source of the packet, the destination of the packet, the cost of forwarding the packet through different routes or paths in the network, or the bandwidth available on different routes in the network. Policy-based routing can also be used to provide a certain Quality of Service (QOS) or Type of Service (TOS) to differentiated traffic in the network. For example, one or more of the various fields (e.g., the TOS bits) in the header of an Internet Protocol (IP) packet can be used by policy-based routers to forward IP packets in a network.
Each policy-based router implements a policy through the use of route maps that define how to forward the packet in the network. Each route map statement or policy statement contains one or more match clauses and a set clause. The match clauses are a series of conditions that are used to determine if an incoming packet satisfies a particular policy. If all of the match clauses of a policy statement are satisfied, the set clause specifies how the router should forward the packet in the network. If one of the match clauses of a particular policy statement is not satisfied, then the policy-based router investigates subsequent policy statements.
FIG. 1 shows exemplary processed policy information 100 of an incoming packet to a policy-based router. Policy information 100 includes several policy fields 102 including a destination address (DA) for the packet, a source address (SA) of the packet, protocol type (PTCL) such as those defined by for an IP packet header, TOS, and COST. Policy information 100 may be received by a policy-based router that implements a policy such as policy 200 shown in FIG. 2. Policy 200 includes three separate policy statements 201 through 203. If policy information 100 satisfies the match clause (i.e., the “if” clause) of one of the policy statements, the set clause (i.e., the “then” clause) of that policy statement determines routing information for the packet in the network. For example, if the destination address of the incoming packet is DA1, the source address is SA1, and the TOS field of the packet is TOS1, then routing information RI2 should be selected.
A policy-based router can use a content addressable memory (CAM)-based system to implement a filtering or classification function to determine whether an incoming packet matches a policy statement. FIG. 3 shows one example of a system 300 that implements policy-based routing using a ternary CAM 302. The policy statements or policy words 201-203 are stored in separate rows in ternary CAM array 304. A ternary CAM array is one that is able to mask entries in a CAM array on a bit-by-bit basis. Ternary CAM array 304 has rows of CAM cells 305 for storing policy field information, and corresponding rows of mask cells 310 for storing mask data. Routing information RI0-RI2 is typically stored in an external memory 308 at addresses corresponding to those at which the respective policy words 201-203 are stored in ternary CAM array 304. Each policy field that corresponds to a match clause for a given policy statement is unmasked by having its corresponding mask bits set, for example, to a logic zero. Conversely, each policy field that does not have a match clause for a given policy statement is masked by having its corresponding mask bits set, for example, to a logic one.
When an incoming packet is received by a policy-based router, it is processed to determine the policy field information. The processed policy field information is provided to system 300 as policy search key 307. For each policy statement in CAM array 304 that matches the policy search key, the corresponding match line ML0-MLN will be asserted and provided to priority encoder 306. In response to the match lines, priority encoder 306 outputs the address of the highest priority matching entry in CAM array 304 to HPM bus 312. If there is more than one matching policy statement in CAM array 304, priority encoder 306 determines that the highest priority matching policy statement is the one stored at the lowest logical address of CAM array 304. For example, as shown in FIG. 3, if CAM array 304 is loaded with policy statement 203 at address zero (i.e., the lowest logical address), statement 202 at address one, and statement 201 at address 2, and a policy search key of DA1, SA1, PTCL1, TOS1, COST1 is provided to CAM array 304, then each of policy statements 201-203 is identified as a match on match lines ML0-M2, respectively. Priority encoder 306 outputs address zero on the HPM bus to select route information RI2 from address zero in memory 308.
Because priority encoder 306 determines the highest priority matching location based on predetermined logical address assignments, policy statements 201-203 are typically preordered or prioritized such that higher priority policy statements are stored in lower logical addresses of CAM array 304 than lower priority policy statements. A policy statement has a higher priority than another policy statement when the route information for the first policy statement is to be selected over the second policy statement even though both policy statements may match the policy search key (e.g., with masking). The prioritizing of the policy statements is typically performed by table management hardware and/or software, which adds overhead to the router. Further, when a policy is changed by adding a new policy statement that has a higher (or equal) priority than at least one of the policy statements already stored in CAM array 304, the table management hardware and/or software often must reprioritize or reorder all or part of CAM array 304. This is typically involves loading the CAM array with the new policy statement and reloading the CAM array with all the policy statements of equal or lower priority. This can add significant overhead to the router (e.g., delay and additional hardware and software) to change even just one policy statement in a given policy. If the CAM array is not loaded correctly, either upon initialization or upon change, addition or removal of a policy statement, an incorrect route may be selected from memory 308.